1. Field of the Invention
The present invention relates to an ultrasonic sensor.
2. Description of Related Art
Lately, an ultrasonic sensor of this kind is installed in, for example, an automobile (vehicle). The ultrasonic sensor transmits an ultrasonic wave from a transmitting device, and receives the ultrasonic wave, which is reflected by a detection object, using a receiving device, and thereby measures a direction of the object around the automobile and a distance to the object. In this manner, by monitoring a surrounding area of the automobile using the ultrasonic sensor, technical development to promote driving safety is taking place.
For example, the ultrasonic sensor is installed in a rear part of the automobile. An automatic parking assistance system, which assists a driver in parking the automobile by backing it with a collision with a human or obstruction being avoided by employing a back sonar for receiving the ultrasonic wave reflected by the human or obstruction behind the automobile using the ultrasonic sensor and for detecting them, is in practical use.
Furthermore, attention is focused on a receiving element of the ultrasonic sensor. The receiving element has a vibrating part including a piezoelectric substance thin film is formed on a thin film part, which is formed as a thin wall part of a substrate using a MEMS (Micro Electro Mechanical System) technology.
When the receiving element of the ultrasonic sensor is installed in the vehicle with the receiving element exposed to an outside, the distance to the detection object cannot be measured accurately if a water droplet or dirt is attached on a surface of the receiving element. As well, the receiving element may be ruined by a load of external force such as a collision with a pebble.
The ultrasonic sensor, which has a protective structure to prevent destruction due to pollution of the receiving element or the load of the external force, is disclosed in, for example, JP2002-58097A. The receiving element is placed in an aluminum case not to expose the receiving element to the outside, and a piezoelectric vibration detecting element for detecting the ultrasonic wave is attached directly to a waveguide-cum-vibrating plate. Accordingly, the ultrasonic sensor receives the ultrasonic wave using the vibration of the waveguide.
However, in the ultrasonic sensor (e.g., piezoelectric or capacitive ultrasonic sensor) that employs the MEMS receiving element, which has the vibrating part to detect the ultrasonic wave using the vibration of the vibrating part, sufficient vibration cannot be obtained by attaching the receiving element directly to a metal case. Furthermore, in the ultrasonic sensor, in which the MEMS receiving element is employed, the receiving element has the piezoelectric substance thin film of low mechanical strength because of its structure. Therefore, when the receiving element is attached directly to the metal case, the receiving element is easy to be damaged.
When the receiving element is not attached to the metal case, and a space is provided between the receiving element and the metal case, the ultrasonic wave cannot be effectively received.